The DEEPSEA CHALLENGER submersible, used to explore the Mariana Trench.

In the ocean’s deepest abyss, scientists found single-celled organisms the size of a salad plate, crustaceans containing a chemical that could treat Alzheimer’s disease, and hints about the origins of life.

In March, an expedition by the Scripps Institution of Oceanography, filmmaker James Cameron and National Geographic explored the Mariana Trench to glimpse the deepest part of the world — a realm that could be the setting for one of Cameron’s otherworldly movies.

Stretching across the Pacific Ocean near Guam, the trench plunges nearly seven miles below the ocean’s surface, to depths that extinguish sunlight and produce pressure a thousand times greater than that at sea level.

“It’s really a foreboding, alien-like environment,” said Doug Bartlett, a professor of marine microbial genetics at Scripps, who said the animals that survive there are uniquely adapted to extreme conditions. “It’s pretty sterile. The guys that are the winners in this landscape are much like what you’d be likely to find in the desert.”

Researchers viewed those creatures through remote landers lowered into the trench, and through photos and samples collected by Cameron, who piloted a specialized submersible to the Challenger Deep, the deepest portion of the trench.

Cameron, whose cinematic work includes the films “Avatar,” “Aliens,” “Titanic” and “The Abyss,” has maintained a parallel passion for marine exploration, tallying scores of submersible dives for both moviemaking and science.

On March 26, Cameron squeezed into what Bartlett called “an incredible green torpedo” for an eight-hour trip into the ocean depths. It was the first dive to the bottom of the trench in more than half a century, repeating a descent made only once before, by a pair of ocean explorers in 1960. Cameron could not be reached for comment for this story.

Cameron’s dive to the Challenger Deep, along with his earlier descent to the New Britain Trench off Papua New Guinea, and remote landings in the Sirena Deep, another portion of the Mariana Trench, shed light on a world of extreme cold and darkness.

“These are pretty bizarre, exotic life forms,” Bartlett said. “There’s not much of them. We’re near the outer boundaries of where life can exist.”

The organisms that survive there include xenophyophores, giant amoeba that are some of the largest individual cells in existence, at up to seven inches in diameter.

The huge single-celled creatures illustrate how deep sea environments can foster “gigantism,” a phenomena that Bartlett said may be related to intermittent food sources in the deep sea.

“It’s a feast or famine existence, where the food supply is really sporadic,” Bartlett said. “If you’re able to engorge yourself with large quantities of nutrients, then you’re likely to do better than a smaller organism that can only take in a small quantity. So being big, as a scavenger, is a huge advantage.”

Other examples of that trend are the giant amphipods the team discovered. The pale, bug-like crustaceans are found in various deep-sea sites, and typically grow about an inch long. But some specimens collected five miles below sea level in the New Britain Trench grew many times that size — up to seven inches. And they were not only big but voracious.